> Paul,
>
> It is not correct that range has to suffer by burning ethanol. In
> fact range can increase if you fly above 10,000 ft density altitude.
>
> The reason is that ethanol has an oxygen content of about 25
> percent. The effect is like supercharging, or nitrous oxide, or
> dragsters that use nitromethane which has huge amounts of oxygen.
> Gordon
Sorry, this is not correct -- the oxygen in ethanol is representative of
the fact that is has been partially oxidized, in other words, partially
burned. So some of the energy content has been expended over being a
hydrocarbon. Nitromethane is a different story. The oxygen can come free
of the nitro-groups and combine with any available hydrocarbons.
You carry less energy per pound with ethanol, how you use it determines
your specific fuel consumption, milage, etc.
The evaporative cooling effects and octane effects are real, but the
oxygen content has already been used up (i.e. not available).
Bill Schertz
Bill,
Thanks for that insight.
What exactly do you mean by, "partially oxidized?" The way I
understand it pure ethanol has an oxygen content of 35 percent, so
all of that oxygen should be available for oxidization, no? (AGE 85
is only 85 percent ethanol -- the rest is pentane isomerate -- so
its oxygen content is about 30 percent.)
The oxygen is the reason the stoichiometric air-fuel ratio of
ethanol is lower than gosoline -- because when you add the same
volume of air, there is more total oxygen due to the oxygen embedded
in the fuel, so you need more fuel to completely oxidize the extra
oxygen. See the attached table, "Ethanol Gasoline Properties."
The heat energy of ethanol per pound or per gallon is lower than
gasoline, as you pointed out. However, the oxygen in the ethanol
results in better thermodynamic efficiency, so power actually goes
up. You can see in the attached chart that an O-470 engine running
on AGE85 has 15 percent more peak power.
The graphic on flight test data shows that at 10,000 feet a Mooney
with an IO-360 running AGE85 has a fuel burn at lean mixture that is
less than the same plane on avgas at 8,000 feet. One of the
advantages of ethanol is that very lean mixtures can be safely run.
Regards,
Gordon.
Gordon,
Thanks for all the background charts. I need to study them in detail,
and am traveling without availability of printer at this time.
Here is where I am coming from. You buy fuel to put energy into your
tank to take with you on the trip. Your engine converts the fuel into
heat by burning it, converting the carbon in the fuel to CO2 and the
hydrogen in the fuel to H2O. The engine then converts the thermal
energy into mechanical energy which drives the prop and then your
airplane.
When you buy ethanol, you are buying a fuel with 11,600 Btu in each
pound of fuel, as opposed to gasoline at 18,700 Btu per pound. This
means that you are carrying less energy with you when you depart the
ground. Reason dictates to me, that if the engine operates at roughly
the same efficiency that leaving the ground with less energy means that
you will not be able to go as far.
Now as to the comment about ethanol being partially oxidized, let me
draw another analogy.
Charcoal is essentially pure carbon (C). If we start to burn charcoal,
the process is:
C + 1/2(O2) ---> CO, with the release of heat. It then it reacts
further, CO +1/2(O2) ----> CO2 with the release of more heat.
CO is carbon monoxide, and will burn, going in the second step to the
fully oxidized state of CO2. CO is a colorless (poisonous) gas, that
during the turn of the century (1900's) was used as a stove fuel and
lamp fuel in houses. It was produced by partially oxidizing coal, and
then shipping the resultant gas to the customers. CO contains less
energy than C, because it is 'partially burned' carbon. CO2 is fully
burned carbon.
CO is 57% oxygen content, and
CO2 is 72.7% oxygen content. I don't anticipate that anyone wants to
run with CO2 as a fuel even though it is a very highly oxygenated
compound.
Ethanol is C2H5OH, and can be made (chemically) by partially oxidizing
Ethane
C2H6 + 1/2(O2) ---------> C2H5OH + heat
Ethanol will then contain less BTU/pound than the Ethane that you
started with because you released heat by partially burning it.
As I stated, I need to study your attachments when I can print them
out and view them in detail. I will do so in about a week.
Keep in mind that ENERGY is what gives you range in an airplane,
and POWER is what gives you performance. Ethanol can allow an engine
to develop more POWER, but it contains less energy/unit weight or volume.
Bill Schertz
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